Pipe Bend Radius

8/10/2019 Pipe Bend Radius

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Close Radius Pipe Bending and Forming

1.0 SCOPE

This specification covers machine 1½D and 3D pipe bends (made at ambient temperatures).Substituting pipe bends for butt welded elbows will provide piping with higher integrity, fasterfabrication, and lower cost.

2.0 BEND RADIUS

2.1 General

Pipe bends are classified according to the centerline radius (CLR) of the bend as a ratio to thenominal pipe diameter. For example, 4" N.P.S. pipe which is bent on a 6" CLR is classified as a1½D Bend (1½ times the nominal pipe diameter). When bent on a 12" CLR, the bend isclassified as 3D.

2.2 1½D Bends

Most cold bends are made on a 1½D radius. The major reason for choosing 1½D is that it isinterchangeable with long radius butt weld elbows (a 4" Long Radius 90°Elbow is 6" center-to-face). As a result, drawings do not need to be changed to permit bending. When modifying anexisting pre-formed system, a standard weld fitting can be used or vice versa.

2.3 3D Bends

As an alternative to 1½D bending, when flow restriction may be a concern, 3D bend radii may beutilized. Studies have also indicated potential energy savings using larger bend radii. 3D Bending

may require special design consideration.

3.0 SIZE LIMITS

Cold machine formed close radius bends are currently available as follows:



1/2 - 6" N.P.SSch 5 - Sch 80

Cold close radius bending of Schedule 5 pipe is only possible with carefully selected pipe andtooIing.

4.0 MATERIALS

4.1 Carbon Steels

Carbon steel pipe is suitable for machine cold bending without heat treatment to the bend radiuslimits indicated in the following table.

A-587ERW

A-53 GradeBSeamless

A-53 Grade BERW

A-106 Grade BSeamless

Cold Bending 1-1/2D 3D - 6D 3D - 6D 3D - 6D

Heat Treatment Normalized Hot Finished Hot Finished Hot Finished

Tensile Strength, Min.PSI

48,000 60,000 60,000 60,000

Yield Point, Min. PSI 30,000 35,000 35,000 35,000

Elong. in 2", Min. % 40 30 30 30

Carbon % Max. 0.15 0.3 0.3 0.3

Manganese % 0.27 - 0.63 1.20 Max. 1.20 Max. 0.27 - 1.06

Sulpher % Max. 0.058 0.06 0.06 0.058

Phosphorous % Max. 0.048 0.05 0.05 0.048

Aluminum % Min. 0.02 Not Specified Not Specified0.10 Min.Silicon

Grain Size Fine Grain Not Controlled NotControlled

Not Controlled

ASME B31.3 - 93

Allowable Stress ValuesKIPS / Sq. In.-20 deg. F to 400 deg. F

13.60 20.00 17.00 20.00

4.2 Alloys



Alloy pipe suitable for machine cold bending without heat treatment to 1½D or 3D bend radius.All other materials should be inquired.

ALLOY SPECIFICATION

Stainless Steel A-312, Types 304, 304L,316, 316L, 309, 310,

317, 321, 347

Nickel Alloys Nickel 200, Alloy 400 (Monel®),

Alloys 600, 601, 625, 690 (Inconel®)

Alloys 800, 825 (Incoloy®)

Alloys C-276, B-2 (Hastelloy)

Aluminum 3003, 5083, 6061-T4,

6061-T6, 6063-T6

Titanium Grade 1, Grade 2

Zirconium 702 Grade

Monel®, Inconel®, and Incoloy® are trademarks of Inco Alloys International, Inc. Hastelloy® isthe trademark of Haynes International

5.0 BEND SPECIFICATIONS

5.1 Bends

A pipe bend made to this specification and verified for pressure design in accordance withASME B31.3 shall be suitable for the same service as the pipe from which it is made.

5.2 Out of Roundness

Flattening of a bend, is measured by the difference between the maximum and minimum outsidediameter at any cross section. For 1½D bends, out of roundness shall not exceed 5 percent of thenominal outside diameter for internal pressure, and 3 percent for external pressure. For 3D bends, out of roundness shall not exceed 3 percent. (See FIG. 1)

5.3 Dimensional Tolerances



Degree of bend is to be held to plus or minus 1 degree. Overall spool length will be held to plusor minus 1/8 inch. All centerline-to-centerline or centerline-to-end face dimensions will be plusor minus 1/8 inch. (See FIG. 2)

5.4 Wall Thinning

Wall thinning of 1½D and 3D bends, as measured by the difference between the actual thicknessof the pipe "A" and the minimum thickness "B" on the outside of the bend shall not exceed 18 percent of the actual thickness for 1½D bends and 12 percent for 3D bends. (See FIG. 3)Maximum allowable working pressure should be calculated using the following formula:

For 1 1/2D Bending 1P= 2SE[.82t -c]D

For 3D Bending 1P= 2SE[.88t -c]D

P - Maximum Allowable Working PressureSE - Allowable Stresst - Actual Pipe Wall Before Bendingc - Corrosion AllowanceD - Outside Diameter of the Pipe

5.5 Wrinkling

Light wall stainless coupled with pipe wall variation near the outer limits of mill specificationsmay occasionally wrinkle. (See FIG. 4) In these cases, peak to valley depth will be limited to:

Pipe Size Max. Depth

1/2" - 2" 1/32"

3" 3/64"

4" 1/16"

6" 3/32"

TABLE 1 - CLOSE RADIUS BENDING TABLES

Piping designers should refer to the Close Radius Bending Tables as a guide to maximizing theuse of bending in their piping layout. These minimum dimensional values are established toallow adequate clamping during bending. Closer center-to-center or center-to-face dimensionsare obtained by cutting pipe and/or adding welds as required. Minimum forming dimensions fordegree of bend other than those shown are available upon request.



1-1/2 DIA. BENDING TABLE

PIPE RADIUS 90 DEG. BENDS 45 DEG. BENDS

SIZE R A B C D E F G 1/2"*

1-1/2"5-1/2"

7" 7" 4-5/8" 6" 5-1/4" 6-1/8"

3/4"*

1-1/2"5-1/2"

7" 7" 4-5/8" 6" 5-1/4" 6-1/8"

1" * 1-1/2"5-1/2"

7" 7" 4-5/8" 6" 5-1/4" 6-1/8"

1-1/2"

*

2-1/4" 8" 8" 10-1/4"6-11/16"

6" 7-5/8"8-15/16"

2" 3" 11" 8" 14" 9-1/4"6-1/2"

10-1/2"12-1/4"

2-1/2"

3-3/4"13-3/4"

10" 17-1/2"11-9/16"

7-3/4"

13-1/8"15-5/16"

3" 4-1/2"13-1/2"

12" 18"10-7/8"

9" 12-3/4"15-3/8"

4" 6"16-1/4"

14" 22-1/4"12-3/4"

9-3/4"

15-1/4"18-3/4"

A or D - Plain or Beveled End B or E - Flanged End C, F, G - Center-to-Center

Minimum Center-to-Center, and Center-to-Face Dimensions to allow forming.Some Center-to-Center bends may require a weld due to the plane of bend and/orthe distance from the centerline of the bender mandrel to the shop floor. * 12"Center-to-Center is required for ½", ¾" 1" N.P.S. and 13-½" Center-to-Center isrequired for 1-½" N.P.S., if the plane of bend is below the horizontal plane (due tobender limitations). 1½D pipe bends in 2½" N.P.S. are not available in Carbon

Steel. material. Please contact APEX Piping Systems with questions regardingany minimum forming dimensions at (302) 995-6136. or fax to (302) 995-1257.

3 DIA. BENDING TABLE

PIPE RADIUS 90 DEG. BENDS 45 DEG. BENDS



SIZE R A B C D E F G

1/2"*

1-1/2"5-1/2"

7" 7"4-5/8"

6" 5-1/4"6-1/8"

3/4"

*2-1/4"

1" * 3" 7" 6-7/8" 10"5-1/4"

6" 6-1/2"8-1/4"

1-1/2"*

4-1/2"10-1/4"

9-3/16"

14-3/4"7-5/8"

6-1/2" 9-1/2"12-1/8"

2" 6" 14"10-15/16"

20"10-1/2"

7-7/16"

13"16-1/2"

2-

1/2"7-1/2"

3" 9" 18" 16" 27"12-3/4"

10-11/16"

16-1/2"21-3/4"

4" 12"22-1/2"

19" 34-1/4"15-1/4"

12" 20-1/4"27-1/4"

6" 18"31-1/2"

34" 51-1/2" 21"23-1/2"

30-1/2" 1"

A or D - Plain or Beveled End B or E - Flanged End C, F, G - Center-to-Center

Minimum Center-to-Center, and Center-to-Face Dimensions to allow forming. Some

Center-to-Center bends may require a weld due to the plane of bend and/or the distance

from the centerline of the bender mandrel to the shop floor. * 12" Center-to-Center is

required for ½", ¾" 1" N.P.S. and 13-½" Center-to-Center is required for 1-½" N.P.S., if

the plane of bend is below the horizontal plane (due to bender limitations). Closer

dimensions for 6" N.P.S. bends are available depending on pipe material. Minimum

forming dimensions for ¾" and 2-½" N.P.S. are available upon request.

Please contact APEX Piping Systems with questions regarding

/The formula comes from elementary beam theory. The value of R is the radius of the deflection curve of an otherwise straight run of pipe. It doesn't have anything to dowith formed runs like long radius bends which involve plastic deformation and it doesn't haveanything to do with fluid flow considerations.

In your basic strength of materials you find that a straight beam under load bends into a curve,and in doing so one side compresses a little while the other side stretches a little. If you do the



arithmetic, you find that the strain = r/R where r is the distance from the centroid of the sectionand R is the radius of curvature of the deflection curve. Since stress = strain x elastic modulusyou get your formula by substituting y/R for strain so Sb = Es x r/R which is your formula. /

I am not able to figure out how the r/R can be treated as the strain.

The basic equation on bending being, M/I = f/y = E/R, where f is stress and y is the distancefrom neutral axis. Equating the last 2, R = Ey/f, rewritten as R = ED/2f, D being the OD of the pipe

/The formula simply states that the stress due to deformation of the pipe under applied transverse load depends on the elastic modulus, theoutside radius of the pipe and the radius of the deflection curve. The assumptions are that the pipe is straight and remains elastic when it curves and that the material behavior is linear andelastic. It doesn't account for local buckling under compressive bending stress--you have tofigure that separately--and it doesn't apply to the process of forming elbows or other bends from

straight pipe.If you want to figure what the radius of curvature is, you need to know the loading and supportconditions for the pipe run and the cross-sectional properties for the pipe itself. Again usingelementary strength of materials, the radius of curvature of the pipe is the elastic modulus xmoment of inertia)/ bending moment = Es x I/ M. As a results you get Sb = (Es x r)(M/Es x I) =Mr/I. This is the usual bending stress formula for a pipe run supported at intervals.

I don't have a copy of B31.8 so I don't know why they specified the bending stress is terms of Rrather than the bending moment, M. Could be there are layouts where a radius of curvature isassumed in design, but the radius of curvature isn't necessarily constant for any loading

condition. /I am quoting a calculation on permissible elastic bend limits from the Aramco pipeline standard AES-L-450 referencing ASME B31.4 para 419.6.4 stress values

26-inch OD pipe, 6.35 mm wall, Grade X52

D = 0.660 m, E = 200 000 MPa, SMYS = 358.5 MPa

Design factor 0.72, maximum temperature 77Â°C, minimum tie-in temperature 38Â°C.

_Calculation_:Sh (Hoop stress) = 0.72

SMYS = 258.1 MPa

St (Temperature stress) = 2.34 (77 -

38) = 91.3 MPa

Sc (Max. combined stress) = 0.9 *

SMYS = 322.6 MPa



Sb (Max. bending stress) = Sc - 0.7 Sh -

St = 50.7 MPa

R (Min. bend radius) =

ED/(2Sb) = 1300 m

Angle per 30 m = 30 /(2R) = 0.0115

radians = 0.66 deg

Allow for misalignment = 0.0044

radians = 0.25 deg

Permit change of slope = 0.0071

radians = 0.41 deg

Max. change of slope 0.0071 x 30000

/BTW, this thread is a really great example of the confusion results from picking an equation out of the blue and plugging in numbers without understanding whatthe equation means or why it's in the Code. The practice of hunting around for equations withoutthinking critically about what they're doing eventually results in a huge bite on the ass./

I have another one used for off-shore pipe laying by stringing off a barge. It also uses the sameapproach.

I note that the calcs do not account for the compression buckling (wrinkling).

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Pipe Bend Radius